The present invention relates to a process for the continuous production of thermoplastically processable polyurethane elastomers by a multistage prepolymer/belt process.
Thermoplastic polyurethane elastomers (TPUs) are well known. They are important industrially due to a combination of high-grade mechanical properties and the known cost advantages which result from thermoplastic workability. A broad and varied range of mechanical properties may be achieved by using different chemical structural components. An overview of TPUs, their properties and uses is described, e.g., in Kunststoffe 68 (1978), pages 819 to 825, or in Kautschuk, Gummi, Kunststoffe 35 (1982), pages 568 to 584.
TPUs are prepared from linear polyols, generally polyesters or polyethers, organic diisocyanates and shortchain diols (chain extenders). They may be prepared by either a continuous or discontinuous process. The process known as the belt process and the extruder process are the best-known processes for their preparation and used industrially.
According to British Patent 1,057,018, a prepolymer is prepared from a substantially linear polyhydroxyl compound and excess organic diisocyanate at temperatures of &lt;100.degree. C., cooled and delivered by a metering pump to a mixing head where it is mixed with a certain quantity of a low molecular weight diol. The reaction mixture obtained is placed on a conveyor belt while heating to a temperature of up to 130.degree. C. The reaction product is then size-reduced and heated. The long reaction time of 1 to 2 hours required for prepolymer formation is an economic disadvantage of this process.
In the extruder process, which is described for example in German Offenlegungschrifts 1,964,834 and 2,059,570, the reaction components ( i.e., the polyhydroxyl compounds, the diisocyanate and the low molecular weight glycols) are introduced into the extruder. They are individually premixed or directly introduced into the extruder as a preadduct of polyhydroxyl compound and diisocyanate and the reaction is carried out in the extruder under certain process conditions. The thermoplastic polyurethane thus produced is extruded in the form of a strand, cooled and size-reduced. The disadvantage of TPU's produced by this process is that their properties are not of a uniformly high standard for all applications. TPU's of the same composition produced by the extruder process are transparent while TPU's produced by the belt process have an opaque appearance. Opaque TPU's may readily be processed to films while transparent TPU's are less suitable for this purpose due to their tendency towards blocking.
Accordingly, there is still a need to find an economic process by which it would be possible to produce both hard injection moldings and flexible films selectively and under controlled conditions.